Satellite television systems generally include a low noise block converter (LNB) at a satellite dish for controlling reception of satellite television signals, and a set top box to control the LNB. Functions of the LNB include downconverting received satellite signals, changing the frequency band of signal reception, changing the signal polarization of reception and, in some applications, controlling more than one receive antenna. For these purposes, the LNB requires control signals and power, both of which are provided by circuitry housed in the set top box located near a television set. A user can change the channel of reception via the set top box. Only a single coaxial cable couples the LNB to the set top box, therefore, the control and power signals are carried by the single coaxial cable.
EUTELSAT, a European organization, which governs television satellite communications, establishes specifications for the low noise block converter control and power signals. Analog AC tone control signals are provided by a 22 kHz, 600 mV peak-to-peak, signal which can be used to implement DiSEgC™ (Digital Satellite Equipment Control) encoding for the purpose of changing the polarization and frequency band of received radio frequency (RF) signals. Power to the LNB circuitry itself is provided by whatever DC voltage is being used to transmit digital control information at any given time. The LNB circuitry requires on the order of 0.6 amp of current.
Adjustable linear amplifiers are conventionally used to provide the control and power signals to the LNB. In this arrangement, the linear amplifier provides an AC modulated output voltage, which has an adjustable DC voltage in response to control signals from a microprocessor within the set top box.
An arrangement that uses a switching regulator to power the adjustable linear amplifier is described in U.S. Pat. No. 7,207,054, issued Apr. 17, 2007, which is assigned to the assignee of the present invention, and which is incorporated by reference herein in its entirety.
Various techniques have been used to protect the set top box from transient signals that can be received from the environment in the cable that connects the set top box to the LNB or in the satellite antenna. Such environmental signals can be the result of lightning or static electricity.
Referring to FIG. 1, a conventional satellite TV system 10 includes a set top box 12 coupled to an LNB 70 though a single coaxial cable 68, coupled to a television set (TV) 64 with a cable 65, and coupled to a remote control 62 with a infrared (IR) link 63.
As described in the above-mentioned U.S. Pat. No. 7,207,054, the set top box 12 can include a switching regulator 16 coupled to receive a DC power input signal 14 and configured to generate a regulated output voltage 16a. The switching regulator 16 can be coupled at its output to a capacitor 18, which can have a large capacitance, for example, about one hundred microfarads. In some embodiments, the capacitor 18 is an electrolytic capacitor. The switching regulator 16 can also be coupled at its output to a capacitor 20, which can have a smaller capacitance, for example, about one microfarad. In some embodiments, the capacitor 20 is a ceramic capacitor. As is known, a ceramic capacitor tends to behave like an ideal capacitor at higher frequencies than an electrolytic capacitor. Thus, the capacitor 16 tend to reduce ripple as may otherwise be generated by the switching regulator 16, and the capacitor 20 tends to reduce higher frequency switching transients as may otherwise be generated by the switching regulator 16.
An adjustable linear voltage regulator 30 is shown here as a field effect transistor (FET) 32 in parallel with a parasitic diode 34. The parasitic diode 34 is not a separate diode, but is a diode inherent to the structure of the FET 32, and is shown in phantom lines accordingly. While the adjustable linear regulator 30 is shown as a FET, it will be understood that an adjustable linear regulator can be designed with a variety of circuit topologies and a variety of circuit components.
The adjustable linear regulator 30 can have an input node 30a coupled to a drain of the FET 32, an output node 30c coupled to a source of the FET 32, and a control node 30b coupled to a gate of the FET 32.
The source of the FET 32 can be coupled to a parallel combination of a diode 54, two capacitors 56, 58, and a zener diode 60, each of which can terminate to ground. In some alternate embodiments, the zener diode is instead a transient voltage suppressor (TVS)
A diode 40, referred to herein as a protection diode, having an anode and a cathode, can be coupled such that the anode is coupled to the source of the FET 32 and the cathode is coupled to the capacitors 18, 20.
The source of the FET 32 is coupled to a connector 66. A signal 32a to and from the set top box 12 is carried by the cable 68. The signal 32a is received by the LNB 70 as a control signal.
The signal 32a also contains television information that travels from the LNB 70 to a TV receiver 52 within the set top box 12. The TV receiver 52 is configured to generate a TV signal 52a carried on the cable 65.
The set top box 12 can also include a voltage divider 24 coupled to receive the regulated output voltage 16a and configured to generate a divided signal 24a. A switching regulator controller 22 can be coupled to receive the divided signal 24a and configured to generate a control signal 22a to control the regulated output voltage 16a of the switching voltage regulator 16. Exemplary switching regulators and control thereof are described more fully below in conjunction with FIGS. 4 and 5.
The set top box 12 can also include an IR sensor 50 coupled via the IR link 63 to receive an IR control signal 62a from the remote control 62 and configured to generate a control signal 50a. A microprocessor 48 can be coupled to receive the control signal 50a and configured to generate a voltage level control signal 48a and a tone control signal 48b. 
The set top box 12 can include a linear regulator controller 46 coupled to receive the voltage level control signal 48a and coupled to receive the signal 32a as a feedback signal in a control loop that controls the adjustable linear regulator 30. The linear regulator controller 46 is configured to generate a linear regulator control signal 46b and a corresponding reference signal 46a. The set top box 12 can also include a tone generator 26 coupled to receive the tone control signal 48b and configured to generate a tone signal 26a. 
The set top box 12 can also include a summing circuit 44 coupled to receive the reference signal 46a and coupled to receive an offset signal 42a generated by a voltage offset generator 42. The summing circuit 44 is configured to generate a sum signal 44a, which is received by the switching regulator controller 22 as a voltage reference signal to control the regulated voltage output signal 16a. 
The set top box can 12 also include another summing circuit 28 coupled to receive a tone signal 26a, coupled to receive the linear regulator control signal 46b, and configured to generate another sum signal 28a. The FET 32 is coupled to receive the sum signal 28a the gate and the adjustable linear regulator 30 is configured to provide a voltage drop controlled by the sum signal 28a. 
It should be understood that the set top box 12 includes two voltage control loops. A first control loop is coupled around the adjustable linear regulator 30 (comprised of signals 32a and 46b to and from the linear regulator controller 46). A second control loop is coupled around the switching regulator 16 (comprised of signals 16a and 22a to and from the switching regulator controller 22). The second control loop is influenced by the first control loop via the reference signal 46a. In other words, the switching regulator loop is controlled by the adjustable linear regulator loop to maintain the regulated output voltage 16a a predetermined number of volts (determined by the offset voltage generator 42), for example, one volt, above a DC voltage drop through the adjustable linear regulator 30 (i.e., from node 30a to node 30c).
In operation, the linear regulator controller 46 is controlled by the microprocessor 48 and also by the feedback signal 32a to generate the linear regulator control signal 46b to control the signal 32a to be selected one of about thirteen volts or about eighteen volts. The microprocessor 48 also controls the tone generator 26 to generate the tone signal 26a, for example, a 22 kHz, 600 mV peak-to-peak, tone signal, which can be used to implement DiSEgC™ (Digital Satellite Equipment Control) encoding for the purpose of changing the polarization and frequency band of received RF signals. The sum signal 28a contains both the tone signal 26a and the linear regulator control signal 46b with a selected DC voltage level.
Via the divided signal 24a and via the sum signal 44a (a reference signal), the switching regulator 16 is controlled by the microprocessor 48 to maintain its regulated voltage 16a at a voltage level in accordance with the selected voltage in the reference signal 46a. For example, when the reference signal 46a is about thirteen volts, the regulated output voltage 16a can be about fourteen volts and when the reference signal 46a is about eighteen volts, the regulated output voltage 16a can be about nineteen volts.
As a result of the above, the signal 32a carried on the cable 68 contains a DC level of either about thirteen or about eighteen volts and also a selected tone signal corresponding to the tone signal 26a. The combination of DC level and tone frequency results in commands to the LNB 70 to tune to one of a plurality of TV channel frequencies. In response, the part of the signal 32a that travels from the antenna 72 to the TV receiver 52 contains one TV channel signal.
When subjected to an external undesired signal 74 (also referred to herein as a transient signal), for example, a signal due to lightning or static electricity, which is coupled to the cable 68 by direct, capacitive, or by inductive means, an undesired current 38 (also referred to herein as a transient current signal) flows through the diode 40 and an undesired current 36 (also referred to herein as a transient current signal) flows through the parasitic diode 34. The diode 40 can be a high capacity diode intended to pass the bulk of the overall transient current, discharging the transient current primarily into the large capacitor 18, and less so into the smaller capacitor 20. However, if the transient current 36 has sufficient magnitude, the transient current 36 can cause the FET 32 to fail by damaging the parasitic diode structure 34. It will be understood that, if the transient currents 36, 38 were not allowed to discharge into the capacitor 18, a voltage would be generated on the cable 68 that could destroy other circuits, for example, more of the set top box 12 or the LNB 70.
It would be desirable to provide a different arrangement that can cause the set top box 12 to survive the undesired signal 74 without damage.